Low-dose aspirin may block the growth and spread of breast cancer, new research suggests.

The painkiller appears to prevent the creation of resistant stem cells that drive the disease.

Scientists made the discovery after laboratory tests showed that aspirin directly and indirectly suppressed the proliferation of two different breast cancer strains.

One was so-called "triple negative" breast cancer which is immune to many treatments.

Aspirin also boosted the effect of tamoxifen, a widely used treatment for the more common form of the disease stimulated by the hormone oestrogen. As well as putting a brake on breast cancer in lab dishes, aspirin significantly reduced its growth in mice.

Anecdotal evidence had previously shown that breast cancer is less likely to return in women taking aspirin to lower the risk of a heart attack or stroke, but the reason why was not clear.

The research showed that aspirin may interfere with the generation of highly aggressive stem cells by tumours. In the mouse studies, treated cancer cells produced only partial or no stem cells. The findings were presented at the annual meeting of the American Society for Biochemistry and Molecular Biology in Boston.

Lead scientist Professor Sushanta Banerjee, from the University of Kansas in the US, pointed out that cancer stem cells are not destroyed by first-line chemotherapy treatments. "If you don't target the stemness, it is known you will not get any effect," he said. "It will relapse."

Aspirin has also been shown to reduce the risk of oesophageal and prostate cancer.

The drug's ability to target a number of different metabolic pathways is what makes it potentially useful as a cancer treatment, said Prof Banerjee. "Cancer is not a single-gene disease," he said. "Multiple genes are involved." However, the researchers warned that excessive use of aspirin can have unwanted effects such as gastro-intestinal bleeding and ulcers.